It is known to form a wavelength selective optical waveguide coupler by locating a pair of optical waveguides adjacent each other so as to provide a set of regularly spaced apart optical coupling regions with the optical path length between the coupling regions being greater in one waveguide than the other, which optical path lengths are a function of wavelengths of the light propagating in the waveguide. Light propagating in one waveguide will couple varying amounts of light into or out of the other waveguide at each coupling region dependent on the relative phase relationship of the light in the two waveguides. The aggregate effect for light propagating through the coupling regions will be additive only for a wavelength in which the optical path difference between coupling regions is such as to cause a relative phase shift of an integral number of 2.pi. radians along the length of the waveguide between the each coupling region. Such a coupler will couple substantially all light of a preselected wavelength from one waveguide into another whilst leaving the other wavelengths substantially unaffected. Examples of such couplers are disclosed in Patent Application No. GB2152694A.
One of the couplers there discussed comprises first and second slab waveguides constructed in a semiconductor substrate. The distance between the first and second waveguides is varied periodically so as to provide the set of regularly spaced optical coupling regions.
The waveguides are formed in a semiconductor substrate. The position of the first waveguide relative to the other cannot be altered so it not only involves relatively complex manufacturing techniques it is also not tunable.
Another of the couplers there disclosed comprises two optical fibre waveguides. The fibres are wound in opposite senses on respective mandrels of slightly different radii for several turns. The path length of a turn is therefore different for each fibre. Optical coupling regions are formed when the turns of each optical fibre are brought together (after reducing the cladding width if necessary). The turns of each of the fibres when brought together, meet at tangents. Coupling occurs at these tangents, and tuning may be achieved by stretching the fibres on one mandrel by increasing the radius of that mandrel.
A disadvantage of this second known type of coupler is that due to the long path length around the mandrels, accidental path differences due to temperature fluctuations for example are difficult to eliminate, and fractional control of the radius of each mandrel must be achieved to a high level of accuracy in order to accurately tune the coupler to selectively couple the desired wavelength.